Types of Telescopes
The name "telescope" covers a wide range of instruments. Most detect electromagnetic radiation, but there are major differences in how astronomers must go about collecting light (electromagnetic radiation) in different frequency bands.
Telescopes may be classified by the wavelengths of light they detect:
- X-ray telescopes, using shorter wavelengths than ultraviolet light
- Ultraviolet telescopes, using shorter wavelengths than visible light
- Optical telescopes, using visible light
- Infrared telescopes, using longer wavelengths than visible light
- Submillimetre telescopes, using longer wavelengths than infrared light
| Light Comparison | |||||||
| Name | Wavelength | Frequency (Hz) | Photon Energy (eV) | ||||
|---|---|---|---|---|---|---|---|
| Gamma ray | less than 0.01 nm | more than 10 EHZ | 100 keV - 300+ GeV | X | |||
| X-Ray | 0.01 to 10 nm | 30 PHz - 30 EHZ | 120 eV to 120 keV | X | |||
| Ultraviolet | 10 nm - 400 nm | 30 EHZ - 790 THz | 3 eV to 124 eV | ||||
| Visible | 390 nm - 750 nm | 790 THz - 405 THz | 1.7 eV - 3.3 eV | X | |||
| Infrared | 750 nm - 1 mm | 405 THz - 300 GHz | 1.24 meV - 1.7 eV | X | |||
| Microwave | 1 mm - 1 meter | 300 GHz - 300 MHz | 1.24 meV - 1.24 µeV | ||||
| Radio | 1 mm - km | 300 GHz - 3 Hz | 1.24 meV - 12.4 feV | X | |||
As wavelengths become longer, it becomes easier to use antenna technology to interact with electromagnetic radiation (although it is possible to make very tiny antenna). The near-infrared can be handled much like visible light, however in the far-infrared and submillimetre range, telescopes can operate more like a radio telescope. For example the James Clerk Maxwell Telescope observes from wavelengths from 3 μm (0.003 mm) to 2000 μm (2 mm), but uses a parabolic aluminum antenna. On the other hand, the Spitzer Space Telescope, observing from about 3 μm (0.003 mm) to 180 μm (0.18 mm) uses a mirror (reflecting optics). Also using reflecting optics, the Hubble Space Telescope with Wide Field Camera 3 can observe from about 0.2 μm (0.0002 mm) to 1.7 μm (0.0017 mm) (from ultra-violet to infrared light).
- Fresnel Imager, an optical lens technology
- X-ray optics, optics for certain x-ray wavelengths
Another threshold in telescope design, as photon energy increases (shorter wavelengths and higher frequency) is the use of fully reflecting optics rather than glancing-incident optics. Telescopes such as TRACE and SOHO use special mirrors to reflect Extreme ultraviolet, producing higher resolution and brighter images then otherwise possible. A larger aperture does not just mean more light is collected, it is collected at a higher diffraction limit.
Telescopes may also be classified by location: ground telescope, space telescope, or flying telescope. They may also be classified by whether they are operated by professional astronomers or amateur astronomers. A vehicle or permanent campus containing one or more telescopes or other instruments is called an observatory.
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